The present invention relates to a rotor for a superconducting rotating electric machine. More particularly, it relates to a rotor for a superconducting rotating electric machine in which the superconducting field coils are more reliably secured to the rotor.
Due to the very high speeds of rotation of a rotor for a superconducting rotating electric machine, the superconducting field coils of such a rotor are subjected to very high centrifugal forces. Since any movement of the field coils may not only result in their damage but may generate frictional heat which can cause a loss of superconductivity, it is extremely important that the coils be rigidly secured to the rotor.
Japanese Laid Open Patent Application No. 57-166839 discloses a rotor for a superconducting rotating electric machine in which the straight, longitudinally-extending portions of field coils are housed in separate, longitudinally-extending slots formed in the rotor and are secured against centrifugal forces by slot wedges inserted into the slots above the coils, while the arcuate portions of the coils are all housed in a single wide circumferentially-extending slot machined in the rotor. The arcuate portions of the coils are separated from one another by electrically insulating packing, and a retaining ring is shrink-fit over the arcuate portions of the coils to secure them against centrifugal forces. However, the electrically insulating packing between the arcuate portions of the field coils has a coefficient of thermal expansion which is about twice as large as that of the rotor or the field coils. Therefore, while it is possible to rigidly secure the field coils in the slots at normal temperatures, when the rotor is cooled to extremely low temperatures during operation, gaps develop between the field coils and the electrically insulating packing. As the electrically insulating packing is not secured to the slots in the rotor, it is possible for the field coils to move, producing frictional heat which may cause a loss of superconductivity. Furthermore, the use of a retaining ring to secure the arcuate portions of the field coils makes it difficult to inspect and repair the coils at a later time, since the retaining ring is not readily detachable.
An alternative method of securing field coils to a rotor which has been used in the past is to house not only the longitudinally-extending portions of the field coils but also the arcuate portions of the coils in individual slots in the rotor. The arcuate portions of the field coils are held in the slots by wedges, just as are the longitudinally-extending portions. No electrically insulating packing is required between coils, nor is a retaining ring necessary, and thus inspection and repair are made easier. However, with this method, it is impossible to install a previously-wound field coil into the slots in the rotor. Rather, the field coils must be wound inside the slots, which makes their installation extremely time-consuming and expensive.